1. Field of the Invention
This invention relates to a method of generating a semi-pure boron vapor by the temperature-controlled decomposition of a metal boride chosen from those specific materials including yttrium, and the rare earths gadolinium through lutetium (atomic numbers 64 through 71), such as would be useful in the production of a feed gas to create ions and ion-clusters for a variety of applications, the ions and clusters being useful in the manufacture of semiconductors, flat panel displays, metallurgical surface modification, or for the formation of surface layers of boron-containing compounds. The preferable metal-borides have ratios of six and higher (Boride-tometal-ratio), and have been identified as uniquely producing boron vapor preferentially to releasing their metal components, yielding a vapor with extremely high boron-to-metal ratios.
2. Prior Art
Boron is an atomic species commonly used in the fields of ion implantation and plasma processing, for the doping of semiconductors, flat panel displays and for surface property modification in metallurgical and ceramic applications. For any of these applications, the method of producing boron ions begins with a boron-containing feed gas introduced into an ion or plasma source. Ions are then created in a magnetically confined space, extracted by voltage potential differential, and accelerated toward the workpiece at a suitable energy. A magnetic mass filter may be then utilized to select a desired species of ion, and the resultant beam of ions is made to impinge on, and be embedded onto or within the near surface region of a semiconductor wafer or other workpiece. While a variety of atomic species are currently in regular usage by this process, boron has always been relatively difficult to produce with as large an ion flux as other common species, such as phosphorus, arsenic, antimony or nitrogen. The low ion flux is significant because of the high cost of ownership of such equipment. Therefore, increasing throughput and yield is a major concern in this industry.
A cause of the low ion flux is the limitation of known feedstock materials to create the necessary ionizable gas. The element boron is highly refractory and does not readily vaporize at temperatures below 2000.degree. C. Therefore, a gaseous compound of boron, boron tri-fluoride (BF.sub.3) is conventionally utilized as the precursor material. Since this BF.sub.3 gas contains only 25 atomic percent boron, the resulting boron ion beam is proportionally diminished, and it consists of a mixture of species, such as B+, F+, BF+, and BF.sub.2 +, of which the desired boron ions are a minority. In addition, all of the boron halides, including BF.sub.3, are extremely toxic and corrosive, requiring special monitoring and gas handling equipment, as well as substantially increased maintenance expenses.
A variety of methods and materials have been employed to produce a more boron-rich feed gas. Other feed gas candidates have included various boron halides, boranes (boron-hydrogen compounds), and lanthanum hexaboride (LaB.sub.6). For LaB.sub.6, see U.S. Pat. No. 5,162,699, Tokoro et al, incorporated herein by reference. In all cases there have been undesirable characteristics of the compound that either reduce its efficiency for producing a boron ion flux or limit the lifetime for evolution of boron vapor. For example, the other boron halides are no better than BF.sub.3 for producing boron flux for the same reason; i.e. the boron beam is a small percentage of the initially evolved species. The boranes are either unstable and gradually decompose to elemental boron in their storage containers, or tend to be difficult to vaporize.
At elevated temperatures (on the order of 1800.degree.-2300.degree. K), LaB.sub.6 emits boron and lanthanum vapor for moderate time periods, on the order of a few hours, before its output diminishes significantly. At temperatures (on the order of 1800.degree. to 2300.degree. K), LaB.sub.6 decomposes primarily by emission of lanthanum atoms. See Handbook of High Temperature Compounds, p. 175-176, incorporated herein by reference. At sufficiently higher temperatures, typically found when LaB.sub.6 is in direct electrical and thermal contact with a heated electron-emitting filament, the residual boron can be made to evaporate because the vapor pressure of elemental boron is reached. The sputtering action caused by ion bombardment in an ion source can also produce some boron vapor from LaB.sub.6 when it is in electrical contact with a filament cathode. However, there are several deficiencies in producing boron vapor using LaB.sub.6. The preferential loss of lanthanum vapor from the surface results in the formation of an insulating boron-rich layer that cannot be readily vaporized except where the LaB.sub.6 is the hottest. The effect of this is to "seal in" most of the remaining LaB.sub.6, severely decreasing boron vapor output, and resulting in an undesirably short operating lifetime. In addition, the boron-rich interface between the LaB.sub.6 and the filament, which is typically fabricated from tungsten, causes the filament to be converted to tungsten boride, rather than boron vapor, thereby "poisoning" the filament. Thus, the use of LaB.sub.6 as a boron vapor source is limited by the preferential emission of lanthanum, the chemical reaction with the tungsten filament, the gradual conversion to hard-to-vaporize elemental boron, and the very high temperature required to produce any boron vapor at all. This combination of factors results in an unreasonably limited lifetime for boron vapor production.
Boron vapors may also be usefully ion implanted into metals and ceramics for hardening of a surface layer. This treatment may be of value for cutting edges, jewelry, optics and tooling. The complexity of conventional ion implantation equipment with costly mass analysis and scanning systems prevents this treatment from being economically applied to low cost workpieces. A more cost-effective, non-mass-analyzed method of generating boron ions would be advantageous.